A progressive addition lens includes a region used for far vision (hereinafter referred to as “portion for far vision”), a region used for near vision (hereinafter referred to as “portion for near vision”), and a progressive region (hereinafter referred to as “progressive portion”). The portion for far vision is located in an upper portion of the lens in use, and the portion for far vision is suitable for relatively far vision. The portion for near vision is located in a lower portion of the lens, and the portion for near vision is suitable for relatively near vision. The progressive portion is located between the portion for far vision and the portion for near vision, and in the progressive portion a surface refractive power is continuously changed. With a spectacle in which a progressive addition lens is used, replacement or removal is not required for switching between the far vision and the near vision. There is no clear boundary line in the progressive addition lens, and the progressive addition lens is excellent in appearance. Accordingly, the progressive addition lens recently becomes the mainstream in presbytism correction spectacle lenses.
A front surface progressive addition lens whose progressive surface is disposed only in a front surface has been the mainstream in conventional progressive addition lenses. However, with development of an aspherical processing technique, a complicated aspherical surface such as a free-form surface can freely be processed at low cost in a short period of time. Therefore, recently a back surface progressive addition lens and a double-sided progressive addition lens are commercially available. In the back surface progressive addition lens, a front surface is formed into a spherical surface while the progressive surface is disposed in a back surface. In the double-sided progressive addition lens, both the front surface and the back surface are formed into progressive surfaces. Particularly, double-sided progressive addition lenses can realize high optical performance (for example, see WO 1997-019383).
On the other hand, not only the optical performance but also good-looking appearance (hereinafter referred to as appearance performance) is required in spectacle lenses. That a center thickness and an edge thickness of the lens are excessively increased, that a height in a thickness direction from the center of the front surface to a peripheral portion (hereinafter referred to as front height) is not excessively increased, and that a distortion of a facial outline is excessively increased in use are included in the appearance performance. That is, in the spectacle lens, even if the optical performance is better improved, usually the wearer does not accept the thick, large-front height lens in which the distortion of the facial outline is increased in use.
Accordingly, in the case where the progressive addition lens for the spectacle is designed, it is necessary that the progressive addition lens be designed in consideration of not only the optical performance but also the appearance performance. However, in the conventional technique, the efficient progressive addition lens designing method is not developed from the viewpoints of not only the optical performance but also the appearance performance. In the conventional method for designing the progressive addition lens for spectacles, after the front surface shape is determined from the base curve and the addition power, the back surface shape is determined if needed in order to improve the optical performance, only the lens center thickness is adjusted for the appearance performance.
There is the need for the method for designing the progressive addition lens in view of not only the optical performance but also the appearance performance.